1. Field of the Invention
This invention relates to devices for measuring the depth of coal remaining on the roof or floor of a coal mine, and particularly to devices which accomplish measurement by measurement of backscatter from a radioactive source.
2. General Description of the Prior Art
It has heretofore been known that the distance from the surface of coal to underlying rock may be measured by positioning a radioactive source adjacent to the surface of the coal, and positioning a radiation detector or sensor nearby and measuring the reflected radiation. In this process, gamma photons are emitted by the source and scattered by the coal and rock, redirecting a number of photons back toward the sensor. The lower density of coal does not absorb as many scattered photons as does the higher density rock. Consequently, the more coal between the detector and rock, the greater number of photons which reach the sensor. The exact number is proportional to the coal depth. Ideally, such a device is directly mounted on a coal cutting machine and its cut determined by the depth information obtained from the device. As far as is known, all prior devices of this type have employed a radiation source and radiation detector commonly contained in a single module or housing, which housing contacts a cut coal surface over a relatively large solid plane area. The difficulty with this is that there are often substantial air gaps between the source and detector and coal surface because of irregularities in the surface, and these air gaps produce significant errors in measurement.
Accordingly, it is an object of this invention to provide a gamma backscatter type of coal interface detector which eliminates the above problem and, when mounted on a coal shearing machine, will provide continuous and accurate coal-rock depth-of-interface information.